A GUIDE TO THE INFLUENCE OF BISMUTH ON LEAD-ACID-BATTERY PERFORMANCE - DISCUSSION

A review is given of the literature that deals with the influence of b ismuth on the microstructure, oxygen/hydrogen evolution kinetics and a nodic corrosion of lead and lead alloys with regard to their performan ce in lead/acid batteries. Analysis shows that there is considerable d isagreement as to the effect of bismuth on lead microstructure. For ex ample, the various investigators report an increase, a decrease, or ne gligible change in grain size. In general, it is concluded that the ox ygen overpotential on PbO2 is lowered in the presence of bismuth. The effect is enhanced as the bismuth content is increased. It is postulat ed that the behaviour results from the formation of a mixed oxide, PbO 2. BiO(x). By contrast, cathodic hydrogen evolution is reported widely to be largely unaffected by bismuth. Nevertheless, there is evidence that the reaction is particularly sensitive to the surface characteris tics of electrodes and that these features can induce either a suppres sion or an enhancement of the hydrogen-gassing rate. Many studies have shown that bismuth accelerates the anodic corrosion of lead alloys, e specially at high concentrations of bismuth. At 0.1 wt.% bismuth and b elow, the effect on the corrosion rate is negligible. The authors of t his discussion are of the opinion that much of the conflicting evidenc e in the areas reported is caused by spurious differences in grain str ucture that are introduced by variations in sample preparation, rather than by the action of bismuth itself. In battery-related tests, bismu th has usually been found to exert little influence on performance, bu t there is some suggestion that cycle life is increased. The present b ody of knowledge is insufficient to confirm the correctness of any cur rently specified maximum level for bismuth with respect to a given bat tery design.